1. Field of the Invention
The present invention relates to colour plasma display panels, of the two-substrate alternating type, with improved light efficiency.
2. Discussion of the Background
Plasma panels suffer from a lack of electroptical performance compared with cathode-ray tubes, this being so whatever the production technique employed.
Colour plasma panels of the two-substrate alternating type operate on the principle of an electrical discharge in the gases and they use only two crossed electrodes, laying on different substrates, to define and control a discharge.
FIG. 1 shows such a plasma panel of the prior art. It comprises two substrates of tiles 2, 3, one of which, called the front tile 2, lies on the same side as an observer (who is not illustrated). This front tile 2 carries a first array of electrodes, called row electrodes, only two of which, Y1, Y2, are illustrated. The row electrodes Y1, Y2 are approximately parallel and spaced apart with a spacing py. The row electrodes Y1, Y2 are covered with a layer 5 of a dielectric material.
The second tile 3 or so-called rear tile is on the opposite side form the observer; it carries a second array of electrodes called column electrodes, only five of which, X1 to X5, are illustrated. The column electrodes X1 to X5 are approximately parallel and spaced apart with a spacing px. The spacing px is about one third of the spacing py and may be between, for example, 100 .mu.m and 500 .mu.m depending on the definition of the image.
The two tiles 2, 3 are generally made of glass. They are intended to be joined together so that the row electrodes Y1 to Y2 are approximately perpendicular to the column electrodes X1 to X5. Once they have been joined together, the two tiles 2, 3 define a space 13 which is intended to be filled with gas. The gas used is generally a neon-based gas.
The thickness H0 of the space 13 between the front tile 2 and the rear tile 3 must be as precise as possible, in order to obtain homogeneous discharges.
On the rear tile 3, the column electrodes X1 to X5 are also covered with a layer 6 of dielectric material. The dielectric layer 6 is itself covered with several groups of three phosphor stripes B1, B2, B3 corresponding, for example, to the colours green, red and blue, respectively. The phosphor stripes B1, B2, B3 are approximately parallel to the column electrodes X1 to X5. They have approximately the same spacing px as the column electrodes X1 to X5. One column electrode, for example X1, therefore lies beneath a phosphor stripe B1, approximately in the middle of it.
In general, the rear tile 3 also includes an array of barriers 11 approximately parallel to the column electrodes X1 to X5 and separated by the spacing px. They separate two adjacent phosphor stripes B1, B2. Their height H1 is generally less than the thickness H0 of the space 13 between the front tile 2 and the rear tile 3.
Two electrodes X1, Y1 lying on different tiles 2, 3 can include a discharge in the gas if they are at appropriate potentials. The discharge region has a cross section which corresponds approximately to the area facing the two opposed electrodes X1, Y1.
For the purpose of reducing the voltages to be applied to the electrodes in order to obtain a discharge, it is necessary to cut out holes or recesses Ep1, Ep2, Ep3, etc. in the phosphor stripes B1, B2, B3, in the surface facing between a row electrode Y1 and a column electrode X1. These recesses Ep1, Ep2 confine the discharge.
Conventionally in colour panels, three neighbouring recesses Ep1, Ep2, Ep3, level with the same row electrode Y1 but in three adjacent phosphor stripes B1, B2, B3, are used to form a trichromatic pixel P which can adopt a great number of colours.
The recesses Ep1, Ep2, Ep3 of the same pixel P are therefore aligned with the same row electrode Y1 and are separated by a distance equal to the spacing px.
To improve the contrast, the front tile 2 is often provided with a black matrix 4 in a form of black stripes extending between two row electrodes Y1, Y2. These black stripes 4 generally occupy an area of about half the area of the front tile 2.
The light efficiency of such two-substrate alternating panels varies in the same sense as the thickness H0 of the gas-filled space 13. It will be recalled with the light efficiency is the ratio of the luminance emitted by the panel to the electrical power that it consumes. Depending on the structure of the panel, this efficiency may actually vary between 0.5 and 1 lumen/watt for a value of the thickness H0 of about 100 micrometers.
However, the thickness H0 of the space 13 cannot be increased excessively with respect to this spacing px without running the risk of disturbing the operation of the panel. A discharge initiated at a recess may trigger spurious discharges at neighbouring recesses that should remain unenergized, especially in panels whose barriers are not full-height barriers.
In so-called coplanar panels, in which the discharges are established between two electrodes carried by the same tile, the light efficiency is not sensitive to the thickness of the gas-filled space.
It has already been proposed, in order to reduce the incidence of these spurious discharges, to use full-height barriers. In addition to their role of separating the differently-coloured phosphor stripes, these barriers have a role of confining the discharge occurring at a recess so that it does not induce a discharge at an neighbouring recess that must not be activated. These full-height barriers also serve as spacers between the two tiles. These barriers allow a greater thickness of the gas-filled space than that required with half-height barriers. However, it has been observed that these full-height barriers can impair the proper operation of the panel, particularly when high pixel bright-up rates is needed. These rates are required in television applications. Complete confinement between recesses lying on adjacent phosphor stripes, belonging to the same pixel, results in a lack of circulation of charges in the plasma and/or of ultraviolet photons capable of helping in discharge ignition.
Another drawback of these full-height barriers is that they are difficult to produce very accurately. They are often produced by successive screen-printing operations, and it is difficult to obtain a uniform thickness.